Polycrystalline silicon resistors, also called polysilicon or polySi resistors, have been used in the electronic circuit industry for many years. Depending upon their doping and doping levels, p+, n+, p− and n− polysilicon resistors can be fabricated. P+ polysilicon resistors are extremely advantageous for use in analog circuit designs due to their desirable figures of merits. Typically, p+ polysilicon resistors are used in analog and mixed signal circuit designs because they can provide precise matching of subsequent resistors, a low temperature coefficient of resistance, a low voltage coefficient of resistance, and a low parasitic capacitance.
Although polysilicon resistors are widely used in analog circuit designs, such resistors generally have high sheet resistance tolerances ranging from 15–20%. This means the sheet resistance changes by +/−15 to 20%. In current analog and mixed signal applications, and in order to meet stringent circuit performance requirements, circuit designers are demanding lower tolerances in polysilicon resistors.
In the current state of the art, polysilicon resistors are fabricated by ion implanting dopants into a polysilicon layer during the source/drain (S/D) implant step and/or emitter implant step. The implanted dopants in the various regions are then activated utilizing a rapid thermal anneal process. Next, a dielectric layer such as a nitride is applied to the body of the polysilicon resistor so as to protect the body of the polysilicon resistor from being silicided in a subsequent silicidation step. The ends of the polysilicon resistors are then typically exposed and silicided by employing a conventional silicidation process that includes depositing a metal atop the exposed polysilicon end portions and annealing. A single or two-step anneal process may be used in forming the silicide depending on the type of metal that is deposited. The two-step anneal typically includes a silicidation formation anneal and a silicidation transformation anneal. During the annealing step, the resistance of the polySi resistor typically changes such that the resistance value no longer meets a predetermined and required resistance value.
In view of the above drawbacks with the prior art process of fabricating Si-containing resistors, e.g., polysilicon or polySiGe, particularly the difficulties in controlling the resistance of resistors, there is a need for developing a new and improved method in which resistors, including polySi or polySiGe resistors, can be fabricated and then adjusted post silicidation.